a) Field of the Invention
This invention relates to an improved electrophoretic display comprising isolated cells of well-defined shape, size and aspect ratio, and the cells have internal sub relief structures and are filled with charged particles dispersed in a dielectric solvent.
The display may have the traditional up/down switching mode, the in-plane switching mode or the dual switching mode.
b) Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a colored dielectric solvent. This general type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively are needed to drive the displays. In contrast, an array of thin film transistors (TFT) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages.
EPDs of different pixel or cell structures have been reported in prior art, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026). However, each of these has its own problems as noted below.
In a partition-type EPD, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of the particles such as sedimentation. However, difficulties are encountered in many aspects including the formation of the partitions, the process of filling the display with the fluid, enclosing the fluid in the display and keeping the suspensions of different colors separated from each other.
The microencapsulated EPD has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules are typically prepared in an aqueous solution and, to achieve a useful contrast ratio, their mean particle size is relatively large (50-150 microns). The large microcapsule size results in a poor scratch resistance and a slow response time for a given voltage because a large gap between the two opposite electrodes is required for large capsules. Also, the hydrophilic shell of microcapsules prepared in an aqueous solution typically results in sensitivity to high moisture and temperature conditions. If the microcapsules are embedded in a large quantity of a polymer matrix to obviate these shortcomings, the use of the matrix results in an even slower response time and/or a lower contrast ratio. To improve the switching rate, a charge controlling agent is often needed in this type of EPDs. However, the microencapsulation process in an aqueous solution imposes a limitation on the type of charge controlling agents that can be used. Other drawbacks associated with the microcapsule system include poor resolution and poor addressability for color applications.
An improved EPD technology was recently disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO2/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. The electrophoretic fluid is isolated and sealed in each cell.
The microcup structure enables a format flexible, efficient roll-to-roll continuous manufacturing process for the preparation of EPDs. The displays can be prepared on a continuous web of a conductor film such as ITO/PET by, for example, (1) coating a radiation curable composition onto the ITO/PET film, (2) making the microcup structure by a microembossing or photolithographic method, (3) filling the microcups with an electrophoretic fluid and sealing the microcups, (4) laminating the sealed microcups with another conductor film, and (5) slicing and cutting the display to a desirable size or format for assembling.
One advantage of this EPD design is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of microcup displays are significantly better than any prior art displays including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive which would limit and predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid within a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way, or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid within the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut to almost any dimensions without the risk of damaging the display performance due to the loss of display fluid in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of displays in a large sheet format which can be cut into any desired size and format. The isolated microcup or cell structure is particularly important when cells are filled with fluids of different specific properties such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing or being subject to cross-talk during operation.
In order to achieve a higher contrast ratio, cells formed from wider microcups and having narrower partition walls are preferred since they allow a higher cell opening area ratio (i.e., the cell opening area to the total area) and as a result, less light leaking out through the inactive walls. Although the resolution of the display may decrease as the cell opening ratio increases, using a wider cell (up to about 300 microns) is still one of the most cost effective ways to achieve a high contrast ratio particularly for low resolution and monochrome applications. However, as the opening area ratio increases, the resistance against compression and/or shear forces imposed by, for example, a sharp stylus for a touch screen panel also decreases significantly. Also, as the opening ratio increases, undesirable particle movement such as convection flow inside the cells becomes more significant and the display contrast ratio in fact decreases. The image uniformity of the display also deteriorates if excessive undesirable movement is present.